Back to back double seal



In this dual seal application the rotating seal faces are facing in opposite directions with a barrier fluid circulating between them.

Compared to other methods of installing seals this is the worst possible configuration.

The other dual seal configurations are:

Why do I say that “back to back” rotating dual seals are the worst possible configuration? Here are some of my reasons:

  • This configuration requires a higher barrier fluid pressure between the seals. This means that an inner seal leak will cause a dilution of your product. There will be no visible evidence of this happening unless someone notices a change in the product concentration or tank level.
  • In operation the outboard seal is carrying the higher differential pressure and should be the first seal to wear out or fail. When this occurs the barrier fluid pressure will drop and the inner seal can blow open. In other words, if the seal works as designed, both seals will fail at the same time.
  • High barrier fluid pressures are hard to maintain because of pressure fluctuations and varying system pressures. Water hammer and pressure surges are not that uncommon.
  • A reversing pressure can blow the inner seal open. Seals should shut with pressure. They should not “blow open” when something goes wrong.
  • If a connection in the barrier fluid system is ruptured the inner seal can blow open, dumping the pump contents to the environment. The second seal would be of no use.
  • Note the snap ring holding the inner stationary face against the end of the stuffing box. This ring is missing in just about every application I have ever seen. Without this snap ring, higher process fluid pressure can over compress the inner seal spring force moving the stationary face into the rotating face, causing massive face wear and very high rubbing temperatures.
  • A common version of this seal utilizes spring loaded dynamic O-rings. O-rings should be placed in O-ring grooves, as shown in the illustration. They should not be spring loaded. The Durametallic CRO seal is typical of that spring loaded configuration.
  • This seal is known as the “double fretter” in the sealing industry. It will groove the shaft in two places just beneath the O-rings. See: shaft fretting.
  • This seal is often specified for slurry applications. Centrifugal force will throw the slurry into the inner faces causing excessive carbon wear. The slurry will then pack in front of the moveable face preventing it from moving as it tries to slide forward to compensate for normal face wear, thermal growth, most impeller adjustments and shaft end play.